Get width and height (excluding margin):
var width = $('#target-element').outerWidth();
var height = $('#target-element').outerHeight();
Get width and height (including margin):
var width = $('#target-element').outerWidth(true);
var height = $('#target-element').outerHeight(true);
Set width and height:
$('#target-element').outerWidth(50);
$('#target-element').outerHeight(100);